1. Field of the Invention
This invention relates to down hole drilling motors, such as turbo drills and drills operated by positive displacement motors, and to certain improved features therein.
Down hole drilling motors were first invented 100 years ago. Down hole drilling motors were first extensively tested in the 1920's . They did not find wide spread use until the 1950's when turbo drills began to be used in the Soviet Union. By the early 1960's, it is estimated that 85% of the wells in the Soviet Union were being drilled with turbo drills. Down hole drilling motors have found wide spread use in the United States for drilling directional holes, but they are not widely used for straight hole drilling because of bearing and seal problems. Commercial down hole drilling motors operate at speeds ranging from 300 to 1,000 rpm whereas, roller bits operate most effectively at speeds of 50 to 150 rpm. At high motor speeds, roller bearings fail after periods of about 5 to 15 hours whereas with conventional drilling equipment operating at lower speeds the bearings of roller bits last up to 200 hours. Down hole motors have had substantial problems in design of radial and vertical thrust bearings, lubrication systems, turbine efficiency, housing construction, etc., which have limited substantially the acceptability of down hole motors in petroleum drilling and in other applications.
2. Brief Description of the Prior Art
Down hole drilling motors were patented soon after the advent of rotary drilling rigs in the 1860's. Cross U.S. Pat. No. 174,922 discloses a very primitive turbo drill. Baker U.S. Pat. No. 292,888 discloses a single stage axial flow turbo drill which is similar in some respects to modern turbo drills. Scharpenberg U.S. Pat. No. 1,482,702 discloses one of the earliest multi-stage turbo drills which was the forerunner of turbo drills currently in use. The Scharpenberg turbo drill contained a lubrication system which allowed the thrust bearing to operate in oil or grease. Drilling fluid acting on a floating piston pressurized the lubricant in the system. The bearings in modern turbo drills operate directly in the abrasive drilling mud, resulting in rapid failures, which limit the application of these drills.
Capeliuschnicoff U.S. Pat. No. 1,681,094 discloses a single staged geared turbo drill. These turbo drills were tested extensively in the Soviet Union from 1924 to 1934. The Russians had severe problems with the speed reducers Capeliuschnicoff turbo drill and subsequently changed to the Scharpenberg turbo drill. Several Russian engineers perfected multistage turbo drills during the 1940's and 1950's and by the early 1960's, the Russians were drilling 80 to 90% of their wells with axial flow turbo drills. The Russians licensed turbo drill technology to companies in the United States, France, Germany and Austria. Turbo drills have had a rather limited commercial acceptance and are used primarily in directional wells.
Virtually all down hole drilling motors have four basic components;
1. Motor section
2. Vertical thrust bearings
3. Radial bearings
4. Rotary seal.
The bearings and seals can be placed in a separate package or unit at the motor section and thus can be used on any type of motor (i.e. turbo drills, positive displacement motors, etc.).
There are two basic types of down hole drilling motors:
1. Turbo drills
2. Positive displacement
Turbo drills utilize the momentum change of drilling fluid (i.e. mud) passing through the curved turbine blades to provide torque to turn the bit. Diamond bits are used on most turbo drills because these motors turn at speeds of 600 to 1,000 rpm whereas roller-type rock bits operate effectively only at speeds up to about 150 rpm. Positive displacement motors have fixed volumetric displacement and their speed is directly proportional to the flow rate. There are three basic types of positive displacement motors in use or currently under development:
1. Moineau motor
2. Flexing vane motor
3. Sliding vane motor
These motors have large volumetric displacement and therefore deliver higher torques at lower speeds.
Thrust bearing failures in down hole motors is a problem because of high dynamic loads produced by the action of the bits and by drill string vibrations. One major oil company placed a recorder at the hole bottom and found that dynamic loads were often 50% higher than the applied bit weight. It was found on occasion that the bit bounced off bottom and produced loads in excess of 120,000 pounds when drilling at an applied bit weight of 40,000 pounds. These high loads can cause rapid failure of the thrust bearings; consequently these bearings must be greatly over designed to operate in the hostile down hole environment.
Two types of bearings have been used in down hole drilling motors:
1. Rubber friction bearings
2. Ball or roller bearings
In existing motors, these bearings operate directly in the abrasive drilling mud and usually wear out in 20 to 100 hours. In addition, the rubber friction bearings have high friction and therefore absorb 30 to 40% of the output torque of the turbo drills. The lift of the vertical thrust bearings can be increased by operating at bit weights which nearly balance the hydraulic down thrust thereby removing most of the load from these bearings.
Radial bearings are required on each side of drilling motors and on each side of the vertical thrust bearings. These radial bearings are usually subjected to lower loads than the thrust bearings and therefore have much longer life. Two basic types of radial bearings are used in down hole motors:
1. Marine bearings
2. Roller or ball bearings
Most motors contain marine bearings made of brass, rubber or similar bearing materials. The marine bearings are cooled by circulated mud through them.
Rotary seals are currently the weakest link in down hole motor design. Improved seals would allow the bearings to be sealed in lubricant, thereby increasing their life substantially. Improved seals would allow bits to be operated at higher pressures thereby greatly increasing drilling rate.
There are six basic types of seals that have been tested in down hole motors:
1. Packing seals
2. Face seals
3. Labyrinth seals
4. Radial lip seals
5. Constrictions (friction bearings and marine bearings)
6. Flow metering seals
Existing drilling motors allow drilling mud to continuously leak through the rotary seals by constricting the flow with any of a variety of seals permitting leakage. Sand and other abrasive particles are filtered out of the mud in the rotary seals which results in rapid failure of the seals. Any substantial improvement in turbo drill design will require positive seals which allow no appreciable leakage.